1. Field of the Invention
The present invention generally relates to a fan controlling circuit, in particular, to a fan controlling circuit suitable for a server.
2. Description of Related Art
A server system usually has multiple sets of fans used for cooling. For a current computer system, fan controlling mainly performs the over temperature protection through an embedded controller (EC) on a mother board. The EC detects the temperature of the computer system, and adjusts an operation frequency of a pulse width modulation (PWM) signal according to the detected temperature, so as to control rotating speeds of system fans in the computer system to adjust the temperature.
A server system has multiple sets of thermal sensors to decide the rotating speeds of fans at different positions, for example, if two mother boards of a server share one fan board, the mother boards of the server provide different PWM signals to the corresponding fan board. In the conventional art, the different PWM signals are integrated by a micro-controller. The pulse width modulation signal having the highest operation frequency is used to drive all the fan boards.
However, the micro-controller is expensive. Further, the micro-controller uses a voltage different from that of the fans, so an additional voltage source needs to be provided to enable the micro-controller to operate, which not only makes the design more complicated, but also increases the design cost.